texts: Will the Sun cool only after it left main sequence?

texts:
I thought that the answer is yes, a little bit. As the sun runs out of hydrogen, then it it gets more and more contracted, increasing pressure, temperature, and therefore luminosity. This is supported by this answer saying, about main sequence stars, that:

result is that older stars a slightly hotter and brighter than younger
stars.

However, Wikipedia says

low- and intermediate-mass stars expand and cool until at about 5,000
K they begin to increase in luminosity in a stage known as the
red-giant branch. The transition from the main sequence to the red
giant branch is known as the subgiant branch.

Am I right that this cooling which wikipedia mentions happens after the star left the MS, while on the sub-giant branch? (Not cooling, and then entering SGB, on the way to RGB.)


texts: I understand the expansion of the universe as actually an increase in the ratio of space to matter. Is this a correct understanding?  Otherwise, I don't understand how an infinite structure can expand.


texts: What is the longest and what is the shortest wavelength of light? How fast would a light-emitting object have to recede in order to shift the frequency of light to increase the wavelength from the shortest to the longest? (If there are limits)


texts: Who is Seyfert, and what is "the so-called Seyfert flare"?

texts:
The BBC's Milky Way's centre exploded 3.5 million years ago says:

A cataclysmic energy flare ripped through our galaxy, the Milky Way, about 3.5 million years ago, a team of astronomers say.
They say the so-called Sifter flare started near the super massive black hole in the centre of the galaxy.
The impact was felt 200,000 light-years away.
[...]The flare created two enormous "ionisation cones" that sliced through the Milky Way.

It mentions "The team - led by Professor Joss Bland-Hawthorn from Australia" and "co-author Magma Guglielmo from the University of Sydney" and says "The findings will be published in the Astrophysical Journal."
Right now I'm just asking the following:
Question: Who is Sifter, and what is "the so-called Sifter flare"?

update: @bertieb's comment notes that the BBC has corrected the passage. The new sentence says:

The so-called Seyfert flare started near the supermassive black hole in the centre of the galaxy, they add.

When first published, the article called it a "so-called Sifter flare" and that's what originally inspired this question.


texts: Does 2 merging black holes necessarily make a quasar?

texts:
2 black holes are about to merge to become a larger black hole. Does this mean it'll become a quasar?

From what I know, quasars are supermassive black holes or a collective amount of them. So, if it merges and becomes a supermassive black hole, does this mean that it is a quasar? If the size is smaller than what would be considered as a supermassive black hole would it not be a quasar?


texts: Jupiter is currently shrinking due to the Kelvin-Helmholtz mechanism. Will this mechanism eventually hold or getting slower? If so, at what size of Jupiter will it stop and why? If not, what will eventually happen to Jupiter?


texts: How bright will Sirius be when it's closest to the solar system?

texts: Preferably in terms of percentage. As a layman, visual apparent magnitude means little to me.


texts: Are we sure there are no planets inside Mercury's orbit?

texts: Currently, most explorations about new planets are pointing to area outside Neptune's orbit, but how about inside Mercury orbit? Can we say for certain there are no planets inside Mercury's orbit?


texts:
Does it exist North of the Universe? (Which is probably used for determining coordinates of far galaxies.)
If so, then where it is?
On what is it aligned? (Is it orientated on Earth, Sun, Milky Way, Local group or on something more bigger?)


texts:
I have a question regarding the units conversion from photon counts on a detector to Rayleighs. I am pretty new to the subject but I would like to know more.
The problem is the following :
I know that $1\, \mathrm{R} = 10^6\ \text{photon}\, {\rm s}^{-1}\, {\rm cm}^{-2}$, but that area should I use in order to obtain the rights conversion. I have an example in which the full area of the HST aperture is used (ish 2.4m in diameter) as a reference but what if I have some optical elements, like a slit for spectroscopic observations?
Should I still consider the same area?
In specific I would be happy to have a systematic way of doing the conversion from counts to Rayleighs using the correct reference area.


texts:
I saw something really odd last night that I cannot understand.
Last night I got up around 3am to catch a glimpse of the upcoming Perseids meteor shower, now I know it's not peaking until next week but we have clear skies so I thought I'd take a look. I've seen hundreds of meteor showers, so know what to expect.
Whilst looking at another part of the sky a large shooting star radiated from the Perseus constellation, I just missed it! However, what was left in the sky was most odd, a really long dotted line of what looked like dust trail from the meteor. Usually, these trails stay where there are, and slowly fade away. But this trail didn't fade and very slowly moved across night the sky in an absolutely perfectly straight line until it faded into the horizon.
From the moment I first saw it to when it fully disappeared into the horizon was probably about 30 seconds or more. What did I see?
(Gloucestershire, UK).


texts: Have we sent radio signals to exoplanets that may be habitable?

texts:
I would like to know please whether or not we have sent radio signals to exoplanets that may be habitable.

Thanks in advance !


texts:
Could someone please tell me if this ratio is important in determining star formation rates?

The ratio is : $$\frac{L_B}{L_{H\alpha}}.$$

Additionally, could someone please explain the implication of the ratio to me or direct me to a source?


texts: How to calculate the moon's illuminated fraction tilt?

texts:
I use "Astronomical algorithms" book by Jean Meeus for programming algorithms for the moon data calculations. I've already created a lot of methods but stuck on the calculation of the moon's illuminated fraction tilt.
Here is the example of how the moon tilt looks like (when you switch date the moon disk turns and so does the illuminated fraction).
I've already calculated: illumination (48.1), phase angle (48.4), position angle of the illuminated bright limb (48.5), parallactic angle and etc. So I think I'm pretty close but still can't get the sufficient result.
I think that the tilt depends on the position angle of the illuminated bright limb but the tilt looks almost the same during the day and the position angle changes its value rather drastically.
I would appreciate any assistance.
UPD
I think I've found the right formula for the tilt calculation (page 347, and in the image attached):
ZenithAngle = MoonPositionAngle - ParallacticAngle.

But I'm not sure if I understand where ZenithAngle exactly is. I made a picture where I marked ZenithAngle (ZOC) and AlphaAngle which is actually the angle I need to calculate.
So if ZenithAngle is ZOC then AlphaAngle = ZenithAngle - 90.

Also I created a sample page with angles values, moon illumination and limb turned by the angle.


texts: Extracting kinematics from stellar spectra. Why are template stellar spectra necessary?

texts:
I am trying to understand how to extract stellar velocity and velocity dispersion maps from spectral cubes, but with no success so far. To be more precise, I do not understand why people use stellar spectra templates to do so.

For gas kinematics (e.g., Ha emission), the usual approach is to fit a Gaussian function to each spectrum. Then, the means and sigmas correspond to the line-of-sight velocities and velocity dispersions.

For stellar kinematics, most people use pPXF. To my understanding, in the simplest case, pPFX fits a parametric function (e.g., Gaussian) convolved with a template stellar spectrum to the data. Why is that? I tried to read the pPFX papers but there is no explanation for this. It assumes that the use of a template stellar spectrum is obvious and there is no justification for this. Actually the majority of papers that use pPFX assume that the use of a template is a de facto thing and provide no explanation.

What is the rationale behind using a template stellar spectrum?
Why can't we just use an inverted Gaussian and fit that to the absorption line of each spectrum?

After some more reading, some papers imply (but do not explicitly say it) that the absorption lines of stellar spectra have complex shapes that can't be modeled with a simple Gaussian. But that is also the case for emission lines! Emission lines can be asymmetric due to multiple kinematic components or beam smearing. Yet, nobody uses templates when extracting kinematic maps from them. I do understand that fitting a single Gaussian to an asymmetric emission line can result in loss of information. But why is this acceptable for emission lines and not acceptable for absorption lines?


texts: Why is every year the same number of days despite the gravity in the solar system?

texts: I was just watching a Geoff Marcy talk on YouTube showing how they infer the presence of planets transiting distant stars. The supposed periodicity wasn't always quite regular from what I could see, and so I wondered if there might be something tugging at the planets as they orbit their star. Then I thought of the earth: Surely there are some years when we are aligned with Jupiter so much so that it would exert some sort of perceptible influence on the planet, thus lengthening the year, even if only by some hours or days, considering that the sun is much more massive. And what would happen every few thousand (?) million (?) years when all the planets and even some of the larger asteroids, Kuiper Belt Objects - everything but the kitchen sink - come into alignment? Wouldn't the earth be drawn outwards to some extent, thus making a year perhaps 370 days long (or whatever)? And maybe temperatures would drop with the planet a fraction of an AU farther from the sun? I know that this is a website for the already enlightened, but I would appreciate an answer that even Josephine Bloggs would understand.


texts: Could the duration of some gamma ray bursts be information from outside our universe?

texts:
Could we be in a system where a large mass explodes dispersing matter in all directions until gravity pulls it together with matter from other explosions until the maximum mass limit is reached, causing another  BIG BANG and the perpetual cycle starts again?
Could the gamma rays of unknown sources detected by the Compton gamma ray  observatory  be information  from  space outside our universe (outsidespace). Could a gamma ray of short duration describe a small universe and a long duration a large universe?


texts:
Assume that two Earth-like planets orbited the same star, with elliptic orbits. Eventually, something occurs that caused the two planets to collide.

What would be the outcome of such a catastrophe? I figure that both planets (and any life if it exists) would be destroyed, but exactly how?

Would both planets explode, combine together into a larger planet, or be able to rebound and either reestablish an orbit or be pulled into the sun?


texts: How much telescope do I actually need for astrophotography?

texts:
I've done the 'sparkly curtains behind landscapes' form of astrophotography for years with DSLRs, lately full frame Nikon. I know what I'm doing with the camera, and want to take the next step into incompetence with telescopes to point at stars with a camera attached, but I'm getting conflicting advice on what kit I need.

One group are saying 'basic motorised Newtonian will be fine' - e.g.

https://www.harrisontelescopes.co.uk/acatalog/skywatcher-explorer-130m-telescope.html#SID=34,
https://www.firstlightoptics.com/celestron-astromaster-series/celestron-astromaster-130eq-telescope.html.

Another are saying 'that's not powerful enough, you need a cadiotropic goto like

https://www.picstop.co.uk/goto-telescopes/celestron-nexstar-evolution-6-schmidt-cassegrain-goto-telescope.html?wgu=217_60210_15449648240362_d6add8be16&wgexpiry=1547556824&source=webgains&siteid=60210.

They're obviously very different bits of kit...! And I can see both arguments TBH. Either one is trying to waste my time with inadequate kit, or another to waste my money into his pocket with overspecified kit.

Help....?


texts: If the Sun got larger, but maintained its luminosity, would the Earth get hotter or colder?

texts:
A recent question If the Sun were bigger but colder, Earth would be hotter or colder? asked - if the Sun got bigger and cooler, would the Earth heat up or cool down. I think the answer to that is mainly that it depends on the final luminosity.

However, what I want to know here (hypothetically), is if the Sun got larger and it's effective temperature decreased such that it's luminosity was unchanged; how would that affect the equilibrium temperature of the Earth? I suspect the answer may involve the wavelength dependence of the albedo, emissivity and atmospheric absorption of the Earth.

Another, less hypothetical, way of asking this is, if you put an Earth-like planet at different distances from stars with a variety of temperatures, such that the total flux incident at the top of the atmosphere was identical, how would the temperatures of those planets compare?


texts: Do planetary surface temperatures change in unison in a solar system?

texts:
Are there any known correlations between the changes in planetary surface temperatures in a solar system?

If so, do the farthest planets have smaller albeit correlated changes?


texts: How likely and severe is the threat of a gamma ray burst to earth?

texts:
In the National Geographic article: http://news.nationalgeographic.com/news/2009/04/090403-gamma-ray-extinction_2.html it is suggested that a gamma ray burst likely caused a mass extinction in earth's history.  How severe is the threat of a gamma ray burst to life on earth?  It would be nice to see a chart that considers the threat within the next hundred years as follows (I have no idea what the actual numbers are, hence I am asking):

Severity - Likelihood in 100 years:

Global extinction - 0.0000001%
Major impact - 0.001%
Moderate impact - 0.1%
Minor impact (e.g. deplete ozone layer by 0.01%) - 10%
Observable - 100% ("minor" gamma ray bursts are frequently detected)

Follow up questions are:

Would there be any warning of an impending severe gamma ray burst?
Could we survive in gamma ray burst shelters? (it would appear being underground may be sufficient according to: http://en.wikipedia.org/wiki/Radiation_protection#Shielding_design and http://2013.org/showthread.php/188-How-does-one-prepare-sufficiently-for-gamma-radiation-in-a-shelter-etc)

Something else I found to be shocking is that the Bible appears to predict such an event.  I would rate the predicted event on the major impact level.  I asked about this in the Christian Stack Exchange here:
https://hermeneutics.stackexchange.com/questions/9324/interpretations-of-revelation-168-11/9335#9335


texts:
I've been working on a space game in my spare time, and lately I've been thinking on how to lay out the universe. Though I've searched around and found it hard to get a good view of what the universe looks like. I did find a lot of good sources, though I'm a very visual learner and found it hard to get a good picture of the universe with what I found. I also don't know what the good, reliable, accurate and up-to-date sources were.

So, are there any good visual sources to get a picture of the universe? Preferably a TV-series or a very well illustrated book, or perhaps even computer program.

Edit: I'm looking for a more general view of the universe. Like what do galaxies look like, and what is their scale. As I understand they contain a combination of planets, solar systems, star clusters, ... Though I couldn't get a good picture of how dense they are. Is there a lot of space (relatively) between these solar systems and star-clusters. I found that our solar system is about 4 light-hours big, though are the neighbouring ones average light years or thousands of light-years away? And in the space between them, is it mostly empty or is there space dust?
The same question for whatever our galaxy circles around (if it does) and what is between different galaxies. (And perhaps what else is there of galaxy-size out there?)

I got a vague view of all of this from reading texts, though I feel like I'm missing a whole lot.

Edit 2: I'm not looking for properties of different planets, that's on a too small scale. In the game I'm planning on making a fictional universe (perhaps randomly generated, though not yet decided), but I want the broad layout to be realistic.

Edit 3: I just found this: http://htwins.net/scale2/ This helped me A LOT in getting a sense of the relative size of things.

Thanks a lot!


texts: Error propagation on a 5 order polynomial

texts:
I have a 5 order polynomial equation which gives log of a chi factor required to convert equivalent widths to normalised H alpha luminosity for M dwarfs, (Reiners et al. 2008). I would like to get the error on this chi factor but my error propagation method is giving an error ~ 7 times the chi factor.
The formula is as such; $log(\chi) = a + bT_{eff} + cT_{eff}^2 + dT_{eff}^3 + eT_{eff}^4 + fT_{eff}^5$
which by taking an exponential on both sides gives; $\chi = e^a \times e^{bT} \times e^{2cT} \times e^{3dT} \times e^{4eT} \times e^{5fT}$
Using $f = ae^{bA}$ ---> $\sigma_{f} \approx |f||b\sigma_{A}|$ I get;
$\sigma\chi = |\chi||\sigma_{T_{eff}}|(b + 2c + 3d + 4e + 5f)$
Can someone please help me in where I'm going wrong.


texts: where is the boundary between dark night and grey night

texts:
Is there a clear cut between them?

How long and how bright will the moon be for a dark night?

I googled but could not find a definition for them. People just talk about them empirically?


texts: What would happen if an ice cube is left in space?

texts: Recently I boarded a flight and noticed outside air temperature as -53°C at an altitude of 36860ft (11.23km). I don't know what causes such a freezing temperature in that altitude but was wondering higher altitudes (space) may have even freezing temperatures. Here I got a doubt i.e what happens if an ice cube is left in space? Would it be melting or stay as it is?


texts: Would "layered" radio interferometry work?

texts:
tl;dr - Is splitting up the process of interferometry as shown in the diagram possible, and if so, is it more efficient and/or easier than traditional methods?

I have been doing some research into radio interferometry, and I had a question about it - I know that we can combine the signals from multiple telescopes into one "image" using interferometry, but what if I was able to do that multiple times?

Let me describe this a bit better. Say I have nine dishes. I arrange three of them in an equilateral triangle, and then I do the same with the other 6, and then, I arrange those three groups of three into a larger equilateral triangle. Then, I combine the signals from each three of the telescopes, and then I combine the three resulting signals. Here's a diagram that might help - the blue dishes are the "telescopes", and the red and green boxes are the "processors" or where the signals would be interfered.

In principle, would this work? And, in the context of radio astronomy, would it be easier to combine or interfere only three signals rather than nine?


texts:
In the past people celebrated important astronomical events and some of those events are still celebrated in some cultures and traditions.
For example: March/September equinox, New Moon, Winter/Summer solstice.

I am looking for a list of astronomical events that people celebrated in the past. Do you know any other important astronomical events that were celebrated in the past?


texts: Why is WISE J0521+1025's distance so uncertain?

texts:
If you look at the Wikipedia page List of nearest stars and brown dwarfs
, you'll notice that all stars mentioned in the list have a very precise value; all are $\pm0.02$ ly (except for GJ 1005, but it is still within 1 ly)
Even though WISE J0521+1025 is within 5 parsecs of Earth, its uncertainty is 1.3 parsecs (4.3 ly)! What is going on here, and why is the distance so inaccurate?


texts: Is gravity a source of infinite energy at a cosmological scale

texts:
As I understand it, gravity cannot be attenuated by any medium (in the way that EM radiation can be, for instance).

Does this, then, not make it a source (theoretically, I am not talking of practicalities) of infinite energy - if we assume the universe itself is infinite and looks the same everywhere (ie there are objects with mass everywhere)?

Or is it more correct to say that gravity has a net contribution of nothing to the universe's energy density because the attractional energy is balanced by a negative potential energy?


texts:
Reading through questions about my favourite planet, Venus, I came across this answer to the question What is the current accepted theory as to why Venus has a slow retrograde rotation?, specifically where the answer suggests one of the possible reasons for the slow retrograde motion as being

  The spin slowed to a standstill and then reversed, caused by the sun's gravity, the dense atmosphere and friction between core and mantle

Based on a fairly recent study published in the Nature article "The four final rotation states of Venus (Alexandre C. M. Correia & Jacques Laskar, 2001).

Given the relatively high density of the Venusian atmosphere, this leads to the question:

How does the dense atmosphere of Venus affect the planet's rotation?


texts: Which stars do I have to use to join with a line to draw constellation?

texts:
I continue working on a planetarium software and I want to draw the lines that the draw the constellation's figure:

Is there a catalogue or something similar with the stars which are the vertex of the figure?


texts: How exactly can the Hubble Space Telescope see up to 15 billion light years?

texts:
I've been researching about the Hubble Space Telescope on nasa.gov, and I've read that Hubble uses a digital camera to take pictures like a cell phone. That's all I've found so far. I can't find any information on how it can see so far away into space. Is the digital camera built to see billions of light years away, or does Hubble have some technology on it that allows it to zoom into the deep space?
If this question has already been answered, please leave the link in the answers.


texts: Aren't there any rock or similar firm material on/in the gas giant planets? What happens if a rock asteroid hits one of these planets? Shouldn't the rock accumulate in the center of the planet due to its high density?


texts: Studies of isotropy from different ends of the visible universe?

texts: Have there been studies to compare structures that are 26 billion light years distant (i.e. from one side of the observable universe to the other) to search for faint differences?


texts: Equation for solar radiation at a given latitude on a given exoplanet?

texts:
I'm trying to find equations that would help me determine the amount of solar radiation hitting a certain latitude on a certain planet given the following inputs:

the degrees of latitude of the location in question
this hemisphere's current season (winter or summer)
size of the planet
luminosity of the star(s) the planet orbits

Without taking into consideration wind, air pressure, or any atmospheric effects.

Ideally I would like to determine the average solar radiation of a given location in both the winter and summer.

My end goal is to determine the average surface temperature of a given latitude on a planet using the base solar radiation and the effects of wind, air pressure, and surface ocean currents.

I know it is possible to do this for the entire planet generally, but I would like some way of doing it for a particular latitude and season.


texts: The Solar Motion and the peculiar velocities of stars

texts:
Local standard Of Rest is the hypothetical origin of a reference system with respect to which the motions of all stars in some neighborhood of the sun average out to be zero

which type of motions of stars average out to be zero and how can I justify that their average will be zero?

What is Peculiar Motion? How does it look like?

How we measure the proper motions of stars as seen from the sun as we can only observe from earth?


texts: Question about tidal forces and the Roche limit

texts:
As we know, an object that is beyond the Roche limit doesn't disintegrate (obviously) because the tidal forces upon the object are weaker than the gravitational pull of the object towards its centre, so the planet remains intact.

My question is: if that's so, why does the body deform even before crossing the Roche limit?

If the gravitational pull of the particles in the object outweighs the tidal forces, wouldn't the object have to stay totally round? There couldn't have been any deformation because the tidal forces are totally cancelled out.


texts: What are the metrics of observation conditions?

texts:
How do professional astronomers measure and describe (ambient) observation conditions?

I know smog, light contamination from the city lights, fog, clouds, rain/snow, humidity etc all contribute negatively to quality of the image, obscuring the celestial objects more or less, but I'm fairly sure there is a more precise measurements of viewing capability for any given location or time than "good weather and far from cities" or "Kinda can see M42 in Orion if you squint."


texts:
Is Earth's moon the only one where a total eclipse of the sun covers the entire sun?

Are any other moons the same size as the sun as viewed from their planet like Earth's moon?


texts: Do planetary magnetic fields slow the movement of conductive extraplanetary moving objects via induction?

texts:
A fixed magnetic field will induce an electrical current in a conductive object moving through the field.  The strength of the eddy currents will depend on the shape and composition of the conductor and also how fast it is moving.  The electrical current produces ohmic heating in the conductor, and some of the kinetic energy of the moving conductor will be turned to heat. This is how induction brakes work.

I wonder if this principle applies to fast moving conductors like asteroids or satellites that encounter planetary magnetic fields.  Are planetary magnetic fields strong enough to act as induction brakes on moving conductors?  A satellite experiencing induction braking would fall to a lower and lower orbit and eventually out of orbit.  Passing asteroids might have their course deviated more than would be expected just from gravity.

My search turned up only this abstract, which attempted to explain why meteorites are hot via this principle.

http://adsabs.harvard.edu/full/1946PA.....54..482R


texts: 22° Sun halo today in Zimbabwe. But there's a moon like thing inside the halo beneath the sun that cannot be seen with the naked eye, what is that?

texts: Guys what is that moon like thing inside the halo?


texts: Change of time in vernal equinox by altitude from the Sea level

texts:
vernal equinox for a very high tower ($h_1>3000m$) in a city ($\lambda , \phi$) in altitude $h_1$ from the sea level is 8:00 Local time. Does this time change if we go to another altitude in that same position ($\lambda , \phi$), for example altitude $h_2$?

In other way, does the time of vernal equinox change with altitude?


texts: Why are larger hex mirrors round and the smaller aren't?

texts: Per this diagram why are the larger (30 meter and European) mirrors filled out to nearly a full circle, while the smaller mirrors composed of hexagons retain a hexagonal border limit instead of filling out and approximating a full circle?


texts: Earth's precession and the position of the Sun in the sky

texts:
To my understanding, the Earth's precession causes an approximately 20-minute difference between the sidereal year and the tropical year. Also to my understanding we use the tropical year for our calendar which means we ignore those 20-minutes. These 20-minutes will cause the calendar to "shift" one day every 72 years so if we don't do anything about it in a few centuries winter and summer will switch places. So every century they add an extra day to the calendar to counter this.
This I understand. What I don't understand is that this 20-minute difference means that the Sun's position in the sky will vary by 20 minutes every year. Meaning if the Sun rises at the right ascension of 0 this year, next year it should rise at the right ascension of 20 minutes, and so on. But each year the Sun will actually rise at the same spot in a particular day.
How is this possible? Where do these 20 minutes go?


texts: Determine Julian Date from Gregorian without formula

texts:
As an exercise, I am trying to calculate the JD from a given date in the Gregorian Calendar at a time given in UT. Furthermore, I want to do so from first principles without relying on any formula. Let's say I choose UT 18:30 on 2/28/2010. I am not able to obtain a result consistent with the U.S. Naval Observatory results. I am in need of a calendar guru to show me where my calculation is going astray.

First I find the number of years from 4713 BCE (Julian Date 0) and the target year making sure to add an extra one so as to include 0 CE. This is $4713 + 1 + 2010 = 6724$ years. The number of these years which are leap is $\frac{6724}{4} - 3 = 1681$. This equation attempts to reflect that every fourth year is a leap year except for those years falling after 1582 (the year of the Gregorian Calendar reform) which are both divisible by $100$ and not by $400$. There are three such years which need to be deducted from the total-- 1700, 1800, 1900. This leaves $6724 - 1681 = 5043$ non-leap years.

Now I convert these to days and subtract the $10$ days which were skipped as part of the reform from October 4 to October 15 in 1582. $5043*(365) + 1681*(366) - 10 = 2,455,928$. Next, I find the number of days from January 1st to the target day in the target year (February 28th). This is $59$ days and so I have $2,455,987$.

Finally, the time number of seconds afternoon on the target day is calculated as a fraction of the number of seconds in a 24 hour period. This works out to be $\frac{6(60)(60) + 30(60)}{24(60)(60)} \approx 0.27$. Adding this to the earlier result yields $JD = 2455987.27$.

The actual answer is $2455255.77$ which leaves a difference of $2455987.27 - 2455255.77 = 731.5$ days. This is nearly two years. Obviously, I have either misunderstood the calculation or have performed it incorrectly. Any help would be much appreciated!


texts: Why did the Comet Shoemaker-Levy-9 fragments cause such large explosions on Jupiter?

texts:
In July 1994, the comet Shoemaker-Levy 9 (D/1993 F2) tidally fragmented and these fragments collided into Jupiter, as per the image below.

Image source

The question is, what mechanisms resulted in these small fragments causing such large explosions in the Jovian clouds?


texts:
I'm looking at stellar objects and doing some math here and...
Now, I can't find these values directly listed, but from it's relative magnitude and distance, (10.27, 471ly,) I calculate T-Tauri to have an absolute magnitude of 4.47, and when I convert that to solar luminosities I get 1.39.
I've run that on objects for which I can find absolute magnitudes and solar luminosities listed, and I'm getting the correct values.
But every description of T-Tauri stars I find says they are brighter than main sequence stars of equivalent mass. A main-sequence star about twice the mass of the sun should be much more than 1.39 times the brightness of the sun.
Now I must turn to people who actually know something about astronomy:  Is the prototype T-Tauri star "weird" in some way, and not actually typical of the class of objects that were named after it, or do I have some bad math or bad data here? What else could be the problem?


texts: Can lunar atmospheric density, as it exists today, increase naturally?

texts:
Are there any mechanisms which could increase the density of the moon's atmosphere.

The moon's existing atmosphere is exceeding rare, with about 1 000 000 molecules of gas per cubic centimeter at the surface, ten trillion times less than the Earth.

The source of this gas is the impact of solar wind on the moon's surface, and some outgassing from the interior of the moon.

Can this density vary significantly? Could it reach a density comparable to Earth's? Are there natural processes which could cause the atmospheric density to increase, either in the short term, or over a longer time span.


texts:
I'm aware that voids are relatively empty regions of the universe, but just how empty can they be? Wikipedia states that voids have 'few or no' galaxies, but I can't find much else.

To make my question specific, suppose I were in the centre of a large void, way outside of the few galaxies that may still exist in this void. Would I occasionally encounter asteroids (if I may call them such), rogue planets, or even some occasional intergalactic star?

Remark: I'm aware that this question is a bit of a one-up of What is there in the intergalactic space?. If this is too much of a duplicate, let me know and I remove the question.


texts: trying to understand stellar nucleosynthesis

texts: Trying to create a periodic table that shows "where" elements are made. For example big bang, cosmic rays, stars, supernovae etc...but reading a lot about gold,and other heavy neutron rich elements, being formed in colliding neutron stars. Is there a definitive list of elements that may have been formed in neutron stars?


texts: Maxwell stress contribution to $\nabla \cdot \mathbf{P}$ in the Navier-Stokes equation for fluid in stars

texts:
I was reading through the following extract outlining how the Maxwell stress contributes to the $\nabla \cdot \mathbf{P}$ term of the Navier-Stokes equation for fluids in a star. Here $\mathbf{P}$ is a general symmetric stress tensor.

I am struggling to see how the estimate $\vert \nabla\cdot \mathbf{P}\vert/\rho \approx 3\times 10^{-3}$ ms$^{-2}$ was derived using the information given in the passage. Can someone please provide a quick derivation of this result?


texts:
I want to "check" Kepler’s first law by using real data of Mars. From the equation of the ellipse, I derived
$$\frac{1}{r}=\frac{a}{b^2}+\frac{a}{b^2}\cdot\epsilon\cdot\cos(\varphi),$$
where $a$ is the major semi-axis, $b$ is the minor semi-axis and  $\epsilon$ is the eccentricity of the elliptic orbit. I'm looking for the following kind of data:

Mars' distance from Sun $r$
the angle $\varphi$ between Mars, Sun and the principal axis of the elliptic orbit.

Then, I want to check, whether $r$ and $\varphi$ fit the measured values of $a$, $b$ and $\epsilon$.
If there is no such data (perpendicular view on Mars' orbital plane) available, how can I transform data given in other coordinate systems to the ones I need? On a NASA website (https://omniweb.gsfc.nasa.gov/coho/helios/heli.html) I found data in "Solar Ecliptic", "Heliographic" and "Heliographic Inertial" coordinates, but I don't know which come closest to my plan.
Update:
I tried it with uhoh's recommendations. Unfortunately I failed.
With the following python code, using the Horizons x, y, z data stored in an xlsx file,
from __future__ import division
import numpy as np
from statsmodels.regression.linear_model import OLS
from statsmodels.tools import add_constant
from statsmodels.tools.eval_measures import aicc
import pandas as pd
import matplotlib.pyplot as plt
horizons = pd.read_excel("horizons2.xlsx")
horizons = np.array(horizons)
horizonsxyz=horizons[:,2:5]
horizonsxyz=np.array(horizonsxyz, dtype=np.float64)
hx=horizonsxyz[:,0]
hy=horizonsxyz[:,1]
hz=horizonsxyz[:,2]

horizonsr=np.sqrt(hx**2+hy**2+hz**2)
horizonsr=horizonsr*6.68459*(10**(-9))

phi=np.arctan2(hy, hx) * 180 / np.pi
phi2=np.mod(phi+360, 360)
phia=np.mod(phi-286, 360)
phiganz=add_constant(phia)
horizonsdurchr=1/horizonsr

horizons_regr=OLS(horizonsdurchr, phiganz).fit()
print(horizons_regr.params)
print(horizons_regr.summary())
y_pred_horizons=np.dot(phiganz, horizons_regr.params)
print(horizons_regr.params)

I get a value of $7.1349\cdot10^{-1}$ for $\frac{a}{b^2}$. This is bad but at least in the right order of magnitude. However for $\frac{a}{b^2}\cdot\epsilon$ I get a really bad value of $-2.89228\cdot10^{-4}$. Deviding the two result yields an estimated excentricity of $0.00044$ which is really far away from the true $0.0934$.
I also tried another approach, using the heliographic data mentioned above. Here, I get closer, but only if I add 35 degrees to the angles, which doesnt make sense, since I should add 74 degrees or subtract 278 degrees, to get the angle relative to the perihelion.


texts: What is the origin of the dust near the sun?

texts:
note: Solar Probe+ is now officially Parker Solar Probe

In https://space.stackexchange.com/q/17498/12102 I asked about the Solar Probe Plus mission. Reading a 2008 report I saw that there is an elevated exposure to high velocity dust near the sun. I understand the high velocity, anything in orbit that close will be moving fast, but I don't understand why there is dust there.
Simply put - wouldn't the gravity pull it in or the solar wind blow it away?
What is the origin of the refractory carbon and silicates? Is this orbiting leftover material from the formation of the solar system, or is it falling in from farther out, or is it left-over carbon and silicon that fell into the early forming sun and is now being blown back out?

4.3.5 Micrometeoroid and Dust. Solar Probe+ will encounter dust particles ranging in diameter from submicron up to several hundred microns and consisting of highly refractory carbon and silicate species with a typical bulk density of ~2.5 g/cm2. The particles will be traveling at relative speeds as high as 350 km/s. To define the shielding requirements for Solar Probe+, a dust model was developed based primarily on the work of Mann et al. (2004). The model employs the following assumptions...

above: screen shot from SolarProbePlus2008

above: screen shot from NASA's Solar Probe Plus Fact Sheet

above: illustration of NASA's Solar Probe Plus from http://solarprobe.jhuapl.edu/spacecraft/ Sun is up - to state the obvious!


texts: What is the current size of the SN1006 supernova remnant?

texts:
I was enamored by the wikipedia article of SN1006.  Esp. the fact that it was so bright, it could cast shadows on the ground.  (Even this idea that you could have possibly read parchment with just the light from the supernova.  Source).

The Question is basically what is the width (radius?) of the remnant as seen in this picture.


texts: Would Venus have any significant effects on Earth if its orbit were entirely within the habitable zone?

texts: If Venus was entirely inside the habitable zone, or life zone, would its proximity to the Earth provoke any remarkable changes to the Earth?


texts: Which kernel does HORIZONS get the Geometric Albedo from?

texts:
I am new to using SPICE and the SPICE Toolkit.  My goal is to extract body radius, albedo, and distance from the sun of a number of solar system objects.  The tools provided by NAIF are great and I have been able to get the information items I need from HORIZONS.  I would like to use mice and the right selection of kernels to construct a table of body information.  I am unable to locate the geometric albedo.  It must be in there somewhere since HORIZONS reports it in its body summary (example below).  Please point me to which kernel(s) the information in the example is extracted from.

Horizons> 500

JPL/HORIZONS                500 Selinur (1903 LA)          2018-Apr-26 15:32:09
Rec #:     500 (+COV) Soln.date: 2018-Apr-05_11:16:59   # obs: 1803 (1903-2018)

IAU76/J2000 helio. ecliptic osc. elements (au, days, deg., period=Julian yrs):

EPOCH=  2455485.5 ! 2010-Oct-16.00 (TDB)         Residual RMS= .28963
   EC= .1453222754778464   QR= 2.232822954662918   TP= 2455876.0720004463
   OM= 290.0072894804399   W=  75.13840817439457   IN= 9.764544808931515
   A= 2.612473556522465    MA= 268.8351702646443   ADIST= 2.992124158382012
   PER= 4.22266            N= .233413627           ANGMOM= .02750885
   DAN= 2.46541            DDN= 2.65631            L= 4.9371039
   B= 9.434865500000001    MOID= 1.25316           TP= 2011-Nov-10.5720004463

Asteroid physical parameters (km, seconds, rotational period in hours):
   GM= n.a.                RAD= 20.414             ROTPER= 8.0111
   H= 9.3                  G= .150                 B-V= n.a.
                           ALBEDO= .202            STYP= n.a.

ASTEROID comments:
1: soln ref.= JPL#41, OCC=0
2: source=ORB

Select ... [A]pproaches, [E]phemeris, [F]tp,[M]ail,[R]edisplay, [S]PK,?,:


texts: how did James Bradley discover light aberration?

texts:
I do not understand his timing method. James
The magnitude of the aberration could be v/c?
The speed of earth is about 30km/s.  That means the deviation from the real position of a star is about 30/300000 rad =180*3600/3.14159/10000 ~20arcsecond.
That is a horrible value.
Could the effect of parallex, proper motion and light aberration of a star be separated clearly?
－－－－－－－－－－－－－－－－－－－－－
Sure. Aberration is always the same. Parallax depends on the distance, and the phase is different.
Why is their phase different? They all have a one year period.
Generally we should not worry about the light aberration, right? Even space telescopes have reference stars. The position of the reference stars are fixed. So although there are detectable wobble(20") in one year , because we use reference stars, this kind of effect is removed every day immediately. Every night, when the night assistant lets a telescope point at some direction, he has to use reference stars to calibrate that night's pointing.
Am I right?


texts: In the end what is the ultimate matter/element in the universe, due to fusion process in stars?

texts:
We know that, in stars, hydrogen is used by the stars and due to the fusion in two hydrogen elements we get helium -> Carbon -> Oxygen ->Other elements -> Iron -> then Super/Hypernova.

If everywhere hydrogen is getting converted to other element then will we(universe) have hydrogen to create/retain stars ?

Or in the end what is the ultimate matter in the universe ?


texts: How to interpret this old degree notation?

texts:
I'm trying to follow the calculations in this paper on page 5 (see image below). The paper is describing traditional methods of calculation for a luni-solar calendar.

The math is not hard, but I can't make sense of the notation.

I figured that the ... = 150 : 1780 (as in step 3) means that 150 is the integer result, and 1780 is the remainder.

But what is ... = 2; 19:28 (step 8)? Degree; minute:second? Okay, but then I'm lost at step 10 with 134 * sin(0:32) = 0:01. I haven't managed to get that result.


texts: Does the Time of Sunset (Sunrise) Change Over Time Given a Fixed Location and Consistent Atmospheric Conditions?

texts:
Assumming a fixed location, clear skies, and consistent atmospheric conditions, does the sunset (sunrise) time change over time?
For example, sunset on February 3, 2021 in Tel-Aviv, Israel, is indicated to be 5:17pm. This is a rounded figure I'm sure. Thus, within a minute, would this stated sunset time have been any different historically---or will it be expected to change in the future?
I know that the length of the tropical year is gradually decreasing overall---does this have any effect on the time of sunset (sunrise)?
Thank you.


texts:
Certain meteorites found on Earth have been established to come from Mars: a giant impact ejected rocks from Mars, these rocks traveled through interplanetary space, and went through the Earth's atmosphere without completely burning up.

Might it be possible to find a meteorite on Mars that comes from Earth from the same kind of mechanism?

Earth's gravity is stronger than Mars', so I guess you would need a bigger impact to send debris into interplanetary space. And the atmosphere is thicker on Earth, so the impacting body would need to be even bigger for enough of it to touch the ground. And you also need to give the rocks more energy so that they can go outwards in the Sun's gravitational well. So I'm guessing that impacts that might be violent enough are rarer for the Earth than they are for Mars.

Might it be possible for there to be Earth meteorites on Mars? And if so, is there any chance that we might stumble upon one?


texts: What caused gravitational waves detected by LIGO?

texts: I understand that in recent discovery gravitational waves have been detected. But knowing space time is not static it probably changes constantly in many places in the universe due to changes in distribution of massive bodies. For example, could the detected wave just be a slight oscillation in Gravity due to Jupiter fly by, or moon, or something similar?


texts: What effects besides "mass defect" cause the alpha ladder beyond iron-56/nickel-56 to be endothermic?

texts:
Many sources state that fusion beyond iron-56/nickel-56 (and certainly beyond nickel-62) is impossible due to them being among the most tightly bound nuclei. For example, in the Wikipedia article on the iron peak (https://en.wikipedia.org/wiki/Iron_peak), it is said that:

For elements lighter than iron on the periodic table, nuclear fusion releases energy. For iron, and for all of the heavier elements, nuclear fusion consumes energy.

However, when you actually compute the mass defect, the alpha ladder would be exothermic up to Tin.
$$ Q=[m(Ni_{28}^{56})+m(He_{2}^{4})-m(Zn_{30}^{60})]c^2 $$
$$ Q=[55.942132022u+4.00260325415u-59.941827035u]m_uc^2 $$
$$ Q \approx 2.709 MeV $$
$$$$
$$ Ni_{28}^{56} + He_{2}^{4} \rightarrow Zn_{30}^{60} (+2.709 MeV)$$
$$ Zn_{30}^{60} + He_{2}^{4} \rightarrow Ge_{32}^{64} (+2.587 MeV)$$
$$ Ge_{32}^{66} + He_{2}^{4} \rightarrow Se_{34}^{68} (+2.290 MeV)$$
$$ Se_{34}^{68} + He_{2}^{4} \rightarrow Kr_{36}^{72} (+2.151 MeV)$$
$$ Kr_{36}^{72} + He_{2}^{4} \rightarrow Sr_{38}^{76} (+2.728 MeV)$$
$$ Sr_{38}^{76} + He_{2}^{4} \rightarrow Zr_{40}^{80} (+3.698 MeV)$$
$$ Zr_{40}^{80} + He_{2}^{4} \rightarrow Mo_{42}^{84} (+2.714 MeV)$$
$$ Mo_{42}^{84} + He_{2}^{4} \rightarrow Ru_{44}^{88} (+2.267 MeV)$$
$$ Ru_{44}^{88} + He_{2}^{4} \rightarrow Pd_{46}^{92} (+2.276 MeV)$$
$$ Pd_{46}^{92} + He_{2}^{4} \rightarrow Cd_{48}^{96} (+3.030 MeV)$$
$$ Cd_{48}^{96} + He_{2}^{4} \rightarrow Sn_{50}^{100} (+3.101 MeV)$$
I ended my calculation here because I wasn't able to find the masses of other isotopes that would, theoretically, follow the chain. I understand that these are highly unstable and their fusion would need an immense amount of energy to overcome the Coulomb barrier. However, my point is that, according to the calculations above, once the barrier is overcome, the fusion would actually release energy, not consume it. So, is the notion of fusion beyond the iron peak elements being endothermic false or am I missing something?


texts: When were the spiders first observed on Mars?

texts: On Mars, there are spider-looking features near the South pole that might be caused by CO2 sublimation. When were these features first observed on the surface of Mars?


texts: Could a star become a planet? I am asking this because the gas giants are ¨Failed Stars¨ and they are classified as planets in our solar system.


texts: A high-temperature isolated neutron star could be super-Eddington?

texts:
If the surface temperature of a young neutron star is hot enough, say, several KeV, its blackbody radiation could be super-Eddington?

Did any paper discuss this scenario before?


texts: How to explain this trajectory of the moon taken from the same spot on Earth over 28 days?

texts: Found this picture online, was wondering why the trajectory has taken such a beautiful shape ?


texts:
I am not an expert, just a enthusiast, so if possible please keep explanation simple.
That said I have start reading about universe expansion and the idea seems to me revolutionary (the image of an infinite bigger universe that inflate).
So I am questioning how researcher validate that specific theory excluding other causes that IMHO would be more intuitive?
For example how they exclude that the expansion is not only a local effect (that's happening on ours patch of observable universe)?


texts: Anyone know if we have any chemical reactions occuring in this quasar?


texts: What is the mean separation between a star and its closest neighbor in the Milky Way halo?

texts: My question is inspired by this previous question, Are there any stars that orbit perpendicular to the Milky Way's galactic plane?. In the great answer posted by @ConnorGarcia, one commenter said he would be really jealous of anybody that could see the view of the Milky Way from close to the galactic poles, since a star stream passes close to them. I think, on the flip side, they would envy us due to their near complete isolation. Space travel outside their home system would be much harder, for example. Here on the galactic plane, near the Sun, I guess the average distance to the closest nighboring star is about ~10 Ly or less. The Alpha Centauri system is about 4 Ly away, for example. Regarding the galactic halo, I guess if we picked a random star there, its closest neighbor would be probably located > 100 Ly away. Is this estimate reasonable? Are there reliable figures on that?


texts: I've recently been seeing a bunch of pictures of nebulae and I'm just fascinated by their beauty and complexity. Is there any kind of telescope that would make it possible to view it from here on Earth?


texts:
My professor asks to me install IRAF for the photometry project but We have a hard time installing it and also I'm more comfortable with the Windows environment (because there is other software that not in Linux that currently installed on my pc). So I'm looking for software that substitutes IRAF and works on windows. It would be a great help.
note: While comments have proposed adding a Linux environment and running there, I'm asking for an IRAF alternative that runs directly in Windows.

I followed the suggestion given by @planetmaker but still some of the things that I do not understand as I'm new to the Linux environment. Hope someone will help me: If you go to IRAF, then I have followed all the step: The last step I have executed is ./install this leads me to

Welcome to the IRAF installation script.   This script will first
prompt you for several needed path names.   Once  the  installation is
complete, you will be allowed to do some minimal system configuration.

For each prompt: hit  to accept the default value, 'q' to quit,
or 'help' or '?' to print an explanation of the prompt.

The next steps from the installation guide, I do not understand it. Can any help me though this?


texts: Were there any images of Sanduleak -69 202 (progenitor of SN1987A) before it exploded?

texts: We all know about SN1987A, the closest observed supernova since Kepler's time. Its progenitor was Sanduleak -69 202, a magnitude 12 blue supergiant, catalogued in 1970. Were there any images of this star before it exploded, and how did we know that this was the star that produced said supernova?


texts: What makes an exoplanet to orbit around its star backwards?And why did we consider our orbital movement to be forward?

texts:
I read somewhere that an exoplanet in the name of HAT-P-7B orbits its star backwards.Why?

And why did we consider our orbital movement to be forward?Is there some scientific reason to consider the movement as forward?


texts: Terminology for empty space at the edge of the observable universe?

texts:
Is there a name for a point at the edge of the observable universe at which there is no visible matter, that appears completely dark?

I'm thinking something like "empty space", but that could equally apply to an empty void between galaxies, which isn't quite what I'm looking for.

So it would refer to a theoretical line in space from the observer to the edge of the universe which contains no visible matter. Is there a term for this?


texts: I'm trying to determine the age of some stars. I have many parameters that characterize these stars: $T_{eff}$ ,log ${g}$ ,$[Fe/H]$, $V$... I've tried to use the isochrones package but so far no success. I was wondering if any of you might have any recommendations!


texts:
As I know, most galaxies are spiral, some elliptical, and a rare type is ring-shaped.

What causes a galaxy to become a specific shape (i.e. ring vs spiral)?


texts: I've been looking around, and can't find a solid answer. Many websites say darkness is the speed of light due to the fact it is the absence of light, but other sources say otherwise, that in some cases darkness is actually faster in some cases. Could someone clarify this for me?


texts: What fraction of the time is EM-L2 in darkness on average?

texts:
This answer to the question How wide is the Moon's umbra and penumbra at EM-L2? calculates the widths of the umbra and penumbra at a distance from the Moon equal to the distance from EM-L2 from lunacenter.

However, EM-L2 spends most of its time in direct sunlight because the Earth-Moon line rarely coincides with the Sun-Earth line.

Taking into account the inclination difference between the Earth's orbital plane and that of the Moon as well as the size of the umbra, roughly what fraction of the time is EM-L2 in total solar eclipse — within the Moon's umbra?

Source


texts: Retrofitting a Dobsonian Reflecting Telescope for smart phones?

texts:
Just received an old Orion Deep Space Explorer 6" Dobsonian Reflecting Telescope.  I'm hoping to gift it to my kids, but I want to add a twist...

We have a microscope with an adapter for an iPhone that has been great fun with allowing time lapse photography and very nice images on large monitors, so I wanted to accomplish something similar with this device.  Telescopes, in the past, have always been a dud due to the lack of multiple kids and family members wanting to be able to see the display simultaneously.

Has anyone had any experience at retrofitting a unit such as this with an adapter that has given good results?  Any gotchas to be aware of?

I'm eying the "HookUpz 2.0 Universal Smartphone Optics Digiscoping Adapter", but a little worried as I've never retrofitted such a device before and would like to hit this spot on since the timeframe is tight for me.


texts: What exactly is a Hamiltonian telescope? Is this one?

texts:
This comment on the current answer to Why is this telescope so short? How hard is it to make such a fast primary? says

In this forum topic Borisov appears to call it an f/1.5 Hamiltonian.

Wikipedia's Catadioptric system; Catadioptric dialytes) says:

Catadioptric dialytes are the earliest type of catadioptric telescope. They consist of a single-element refracting telescope objective combined with a silver-backed negative lens (similar to a Mangin mirror). The first of these was the Hamiltonian telescope patented by W. F. Hamilton in 1814. The Schupmann medial telescope designed by German optician Ludwig Schupmann near the end of the 19th century placed the catadioptric mirror beyond the focus of the refractor primary and added a third correcting/focusing lens to the system.

The image of the telescope in the linked question shows a very short and squat telescope with a 65 cm aperture that does not appear to have a "single-element refracting telescope objective", so I am confutes!
Questions:

What (the heck) is and is not a proper Hamiltonian telescope?
Is the Gennady Borisov telescope in the linked question (and shown here) one?


texts: Why aren't other kinds of energies considered in the mass-energy of the universe?

texts:
Mass-Energy of the Universe:

5% ordinary matter,
27% dark matter,
68% dark energy

What about other energies such as thermal energy, potential energy, kinetic energy, chemical energy, and radiant energy?


texts:
Stars with more than 8-16 solar masses usually go supernova. These are OB stars, of which 0.2% currently exist in our galaxy. The average lifespan of said star ranges from 10 to 40 million years. So 200 million OB stars ÷ 40 million years = 5 per year. That's how many supernovae we should be seeing annually in our Galaxy alone. So why do we see only an average of one supernova every 50 years?
Clearly, I am not considering something else important here. What am I missing?


texts: Using the surface-brightness relation for Cepheids

texts:
I'm a high school student who is doing research about Cepheid variables.

I'm in the process of deriving distance to Cepheid using the Baade-Wesselink method, which utilises the surface brightness relation. The surface brightness relation is used to derive angular diameter variation of the cepheid from its V-K band data in this method. I found the equation relating angular diameter and the V-K band of Cepheid variables from the research paper The projection factor, period-radius relation, and surface-brightness colour relation in classical cepheids, which is apparently:

$$\log{θ}=(0.2672+0.0016)(V-K)+(0.5354+0.0012)$$

My question is, to derive the angular diameter variation what kind of raw data that I should use for V-K band? May I directly use the maximum V-K value and minimum V-K value and put them into the equation individually, and interpret their difference as an angular diameter variation? Or do I need to get lists of V-K band value varying over time and put it into the equation to get angular diameter variation over time and graphically measure the maximum and minimum angular diameter variation?


texts: If rotation has nothing to do with gravity, why do astronomers always link swirling dust and gas to stellar evolution?

texts:
All my life I was taught in school that we have gravity because the earth spins.  Now I know that is not true.  Online academics were baffled why anyone thought so. I can only say that's what they said in school.
But if gravity is more mass and everything in the universe has some gravitational "pull", then why do astronomers always use spinning and whirling dust and gas as a way to explain stellar evolution?  They make it sound like the spinning around created the gravity to form the mass and the bigger it got the more it spun and made more gravity.


texts: Planetary phase angle of Hohmann transfer

texts:
This is Question 23 from USAAAO First Round 2015. The answer is D. Can someone show how to get the answer?

Calculate the planetary phase angle (counterclockwise from Earth, a =
1.0 AU) that a probe may correctly complete a Hohmann transfer orbit to Venus (a = 0.7 AU)

a. 141 degrees
b. 17.5 degrees
c. 121 degrees
d. 241 degrees
e. 343 degrees

What I tried:

Semimajor axis of Hohmann orbit: 0.85 AU
Period of Hohmann orbit: 0.7836 yr
Time of flight: 0.3918 yr
Period of Venus: 0.5856 yr
Angular distance travelled by Venus during flight: 241°

The last one looks suspiciously similar to the answer, but using that angular distance the phase angle should be 61°. Feel free to correct me anywhere that I’m wrong


texts: Sky photography through compositing images in software?

texts:
I've read about and seen some rather sophisticated rigs for night sky photography that allow for ultra-long-exposure photos by counteracting the rotation of the sky by rotating the camera with it. The problem is mechanics for such rigs is rather complex, expensive and not entirely within reach of an amateur.

I was considering a different approach:

Set the camera on a firm tripod or whatever, leaving it motionlessly.
Use intervalometer in the camera firmware to take multiple photos of moderately long exposure, short enough that the stars don't blur into lines.
Save each as RAW so that the trace amounts of light captured in each of them don't get lost.
Either using more bright stars as markers, or even calculating the misalignment of the photos basing on movement of the sky and lens geometry composite the images in software, to obtain good images of the night sky.

Obviously I wouldn't get very far with high zoom images, as the field of vision would shift away long before I'd get enough light in for a good photo, but obtaining a photo of a fair section of the sky from a relatively wide angle lens should be doable.

Is such approach used or even viable? Is there such software "out there" already or would I need to write my own from scratch?


texts: Cosmological redshift vs doppler redshift

texts:
I'm reading Harrison's "Cosmology: Science of the universe" because Harrison focuses on the distinction between cosmological redshift (he calls it expansion redshift) and the Doppler redshift.

He states that "they [Doppler redshifts] are produced by peculiar and not by recession velocities"
and
"[expansion redshifts] are produced by recession and not
peculiar velocities".

I understand the concepts of both kinds of redshifts but have a hard time understanding this strict separation. Please correct me, where/if I am wrong:

Suppose a galaxy has no peculiar movement. This means that its position will stay (approximately) the same in comoving coordinates. In fact though ( proper distance), it IS moving away from Earth with recession velocity V, caused by the expansion of the universe. So it should have a Doppler effect based on this recession velocity AND a cosmological (expansion) redshift should also take effect because the light gets stretched with the expansion of the universe. Even though the recession velocity is not due to a peculiar movement, it means that the source of light is moving away from the observer and hence the light should be redshifted and on top the light gets redshifted on its way through expanding space.

Please correct me or tell me if I am right or wrong, I have spent a lot of time reading but still don't fully understand this. Thanks.


texts: Given that a color index ($B-V$) of star A is lower than that of Star B, What can we say about the stars?

texts:
The color index $B-V$ of the star is defined as
$$B-V\equiv M_B-M_V$$
Let us suppose it's given
$$(B-V)^{(A)}<(B-V)^{(B)}\Rightarrow M_B^A-M_V^A>M^B_B-M^B_V$$
The notations are a bit confusing, but I think you would pardon me.
Now we know that Stellar magnitudes decrease with increasing brightness which means a star with absolute magnitude $-5$ will be brighter than a star with absolute magnitude $-2$.
I'm reading Introduction to Modern Astrophysics By Carroll and Ostlie. The  author says,

A star with a smaller $B-V$ color index is bluer than a star with a larger value of $B-V$.

I don't understand, How??
Star A is bluer than Star B means
$$M_B^A<M_B^B$$
Given four numbers with relation
$$x-y<a-b\not\Rightarrow x<a$$
Then how we can say so??


texts: I am reading .fil File using UCBerkeleySETI / blimpy, but I am getting error?

texts:
I am working on pulsar search and I am reading the filterbank file using blimpy technique in python, but I am getting error.
My question is that blimpy is a universal file reader, like .hf, .raw and .fil. The file I am currently processing is provided in the link below.
I know that presto can process this file using readfil but I am currently trying to write my own code for pulsar search, for this, I am trying to read .fil file using blimpy and then apply other steps.
http://www.cv.nrao.edu/~sransom/GBT_Lband_PSR.fil
from blimpy import Waterfall
import pylab as plt
import numpy as np
import math
from scipy import stats, interpolate
file_path='GBT_Lband_PSR.fil'
obs = Waterfall(file_path)


texts: Is there an advantage to the equatorial region of the far side of the moon for a radio telescope or would any crater on the far side work?

texts: If nearly any crater on the far side of the moon will work, and not just equatorial craters, then we could pick craters near the possible future Artemis base at the South pole which would allow easier access.  My assumption is as long as the crater is outside the range of Earth and the satellites in orbit it would be essentially the same as an equatorial crater.


texts: Thinking about the Twin Paradox, what would a Twin Traveling at .8c, calculate as the amount his EBT aged at the mid-point in a 4 lightyear trip?

texts:
Referring to the twin paradox, where we have an Earth Bound Twin “EBT”  and a Traveling Twin “TT” who travels away from Earth at velocity= .8c, lets say to Alpha Centauri, when the TT is half way there, what would the TT calculate as how much his EBT’s aged at that point?
To simplify, round the distance to Alphas Centauri to say 4 lightyears.

I am thinking .8c yields a lambda of .6 and so the TT’s  4 lightyear trip, reduced to 2.4 lightyears due to space contraction. Therefore the midway point is only 1.2 light years. Traveling at .8c, means the TT must himself be 1.2/.8…. 1.5 years older at the midway point.
Since his EBT, relatively, is “traveling” at v=-.8c during this time, his EBT must appear to age relatively slower compared to him.  Using the lambda of .6, the TT would calculate as the amount aged by his EBT to be 1.5*.6=.9 years.
Is this correct?


texts: What would we notice if the Sun stopped fusing atoms?

texts:
(This question was stimulated by Would we have more than 8 minutes of light, if the sun "went out"?)

According to this answer, the "Sun has enough gravitational potential energy to supply its current luminosity for tens of millions of years."

Does "current luminosity" mean that the same spectrum will be radiated, or will it be just as bright, but radiating at different wavelengths (like not in the violet-blue and reddish orange-red wavelengths which chlorophyll requires, or the IR that keep us warm)?


texts: Which physical quantity is the real/definitive tracer of galaxy evolution?

texts:
Which physical quantity is the real / definitive tracer of galaxy evolution?

Let me explain better: if you aimed at understanding as much as possible about the evolution of galaxies and you had to choose only one quantity to perfectly follow/observe (directly/indirectly) across the cosmic time, which quantity you would chose? why? Is it possible? If not, why? What combination of quantities you would choose?

Stellar Mass, SFR, Metallicity, Dark Matter Mass, Angular momentum...


texts: How does the shape of the gas and shape of stars change as the galaxy ages? Does there exists any such formula which relates the shape/volume with the time?


texts: Is it possible to predict a supernova with years or decades of advanced notice?

texts:
I know star phenomena like solar flares can (at some degree) be predicted:
https://www.dailymail.co.uk/sciencetech/article-5371481/Scientists-predict-solar-flares.html

Question: Is there any phenomena in a star that could be used to predict its supernova explosion with years or decades of advance notice?

Answers to Are there observable changes in a star about to become supernova, minutes or hours before the explosion? address shorter timescales like hours or days, but I'm looking for times that are orders of magnitude longer than those discussed there.


texts: What's the number density of `Oumuamua like objects?

texts:
Recently, the fly-through the Solar system of the inter-stellar object (ISO) `Oumuamua caused quite a stir in the astronomy community. I wonder whether there are reliable estimates for the (local) number density of such objects. Such estimates could be based on the `Oumuamua event (when they are necessarily rather crude owing to very low-number statistics) or on any other constraints.
A related question is: how close to Earth must an ISO pass to be detected with today's instruments, or 10, 25, 50 years ago?


texts: Could images of (very) nearby targets be recovered from Event Horizon Telescope data?

texts: Reading this question I was struck by the fact that the "pointing" of an interferometer is effectively done in the correlator, at least within the common field of view of all the telescopes, which is many thousands of times wider than the imaged area. Could the same data be reprocessed to reveal images of other "tiles" from this common field of view? Could it image the accretion disk, for example, or the base of the jets? Are there any targets that are close enough and bright enough to be worth doing this?


texts:
There were theories that Pluto's atmosphere collapses when it gets too far from the sun.

Then I've read papers that says that it doesn't.

But NASA says that the surface pressure went down recently (implying that it does).

Is there any clear data as to what's going on?


texts: What should I expect to see with my telescope with what EP's?

texts:
Recently got a Celestron 130 SLT (first "real" scope) and have a question. I have the following:

130mm F/5 newtonian, 650mm focal length scope

9 and 25mm factory EP's (more on that later)

Svbony 2x barlow

Very, very dark skies (100 miles from any cities, out in the desert, light pollution maps show "zero light pollution" on the maps anywhere I view from once I'm 5 minutes from home). I don't have expensive glass, I don't know what I'm doing... but at least I have this going for me :-)

4500 elevation with low mountains on either side, 10-25 miles away  (don't know that it matters)

20/20 uncorrected vision (if that makes a difference)

I am curious what I should expect to be able to see with my scope, and at what magnification I should see them at? I plan to buy 3 EP's, and want to know the lowest and highest I should buy to see what I "should" be seeing. I don't want to pursue things that I won't be able to make out clearly anyways, or buy the wrong EP's for seeing those things. I see anywhere from 25 to 60 X your scope size, which is a wide range of mags.

I have beautifully-dark skies, so I'm not sure if the EP has to do with light pollution also, or solely "seeing".

Will I be able to make out cassini's division? Will I be able to complete the messier list?

I have nothing in-person to compare my scope to, so I'm relying on random internet information, which tells me my max magnification is anywhere from 150 to 250, and varying features and objects that I will be able to see. Any input will be greatly appreciated. I plan to start the lunar observing program soon (LOVE the moon, mostly because of my spaceflight interest and occupation), and have only looked at the moon so far, along with learning my constellations.

Thanks!

(PS, whatever EP's I buy, I am willing to spend up to 80 a piece, on 3 total new ones (or 2, the 25mm seems ok)


texts: Specutils Gaussian1D fitter and estimators

texts:
I am running the "simple example" from specutils to do some line-fitting.
The demo code is here:
https://specutils.readthedocs.io/en/stable/fitting.html
This runs and does what it says.
However I want to use parameter estimation to get an approximation to the initial parameters in the Gaussian, because I am interested in looking at a number of long spectra with a lot of lines in and want to attempt initial fits automatically.
So I modify the specutils.fitting import to
    from specutils.fitting import fit_lines, estimate_line_parameters

and add in
    e1 = estimate_line_parameters(spectrum, models.Gaussian1D())
    a = round(e1.amplitude.value,2)
    b = round(e1.fwhm.value,2)
    c = round(e1.stddev.value,2)

This gives meaningful values and I replace the call to the Gaussian by:
    g_init = models.Gaussian1D(amplitude=a*u.Jy, mean=b*u.um, stddev=c*u.um)

then
    g_fit = fit_lines(spectrum, g_init)
    y_fit = g_fit(x*u.um)

The initial values I get from the estimator are:
initial amplitude= 3.35  initial fwhm =  2.41  initial stddev= 1.02
But on looking at the output parameters using g_fit.amplitude.value etc, I get:
final amplitude= -0.24 Jy  final fwhm =  0.0 um  final stddev= 0.0 um
for the output values!! If the estimators are correct then the output values should be very close.
Is there an issue using the estimators like this?
FWIW I am running matplotlib 3.2.2, specutils 1.0, numpy 1.19, python 3.6 over Ubuntu 19.10


texts: On analogies between gas and stellar systems

texts:
Analogies between (typically) ideal gas and stellar systems are not only intuitively valid to some extent, but have been established and used in the studies of stellar clusters and galactic systems, most often as a simplification to collisionless Boltzmann equations.

The idea behind the analogy is that if a stellar system can be represented as a set of point masses, and if the number of point masses is large, then we can consider them from the point of view of kinetic theory of gases. One thing to remember here, though, is that stellar gas system is neither relaxed, nor can be relaxed.

I am curious here: How far can the described analogy be pushed?

For example, there is a range of gas-specific phenomena (or we could be talking about plasma, if you may prefer), which would be fascinating to imagine for stellar systems, such as shocks, turbulence or viscosity. Can such, or some other, characteristic phenomena exist in stellar systems and are there actual systems exhibiting such a behavior? (of the named ones, viscosity analogue exists and is rather common)


texts:
What is the significance of astronomical twilight? Astronomical twilight is when "the geometric center of the Sun's disk is between 12 and 18 degrees below the horizon" , but what, if any, is the significance of that event?
Here's a description of what astronomical twilight is, but I fail to see any mention of the event's significance.


texts: Are black holes considered part of baryonic content of universe?

texts: Are black holes (like those detected by LIGO, for instance) part of baryonic content of universe or part of dark matter? Why?


texts: Where is the Perseids meteor shower coming from?

texts: Swift–Tuttle Comet has orbital period of 133.28 years. Yet, people say that debris and dust from this comet causes this meteor shower. This does not make any sense. Or do they want to say that from 1992, every year, for 28 years Earth picked up debris left behind so many years ago while passing the August part of orbit? It's logical to assume that all this debris would be exhausted by now...


texts: What are shooting stars and how are they formed?

texts:
What are shooting stars? How are they formed and how often do they occur during the night?

Also, why are there more shooting stars on some nights than on others?


texts: Why does the sensitivity to GWs drops off inversely proportional to the distance?

texts:
This answer makes me wonder why the sensitivity to gravitational waves decreases proportionally to the distance.

Since gravitational waves extend in all directions, my (uneducated) guess would be that the same argument can be made as for the decrease in sensitivity for electromagnetic waves. Why is this not the case? Why does the sensitivity to GWs decrease in a linear fashion?


texts: What's the relationship between Moon phase and the Milky Way visibility?

texts:
I've been using an IOS app to predict visibility of the Milky Way.

I'm planning a trip to capture the Milky Way on 9th March 2015, but the app shows the visibility will be very weak on that date.

The Moon doesn't even rise until 9:12am.

Does the Moon phase still affect the visibility of the Milky Way even if it is not above the horizon?


texts: Martian dust storms-what was the smallest in area, or shortest in duration storm detected?

texts: I understand localized Martian dust storms can develop into global wide events. All searches I do produce results that focus on local storms expanding.  Is there any information on what was the shortest duration or smallest area dust storm recorded?


texts: How many tsunamis have been caused by meteorites falling?

texts:
I don't know if this is a better fit for Earth Science (because of the tsunami part) or for Astronomy (because of the meteorite part).
The Wikipedia page on tsunamis says that tsunamis can be caused the the fall of a meteorite. The first reference after that claim mentions that tsunamis can be caused by meteorites, but in an unsubstantiated way. The second reference is behind a paywall so I can't access it.
How many recorded tsunamis have been caused by the fall of a meteorite?


texts: How to get ellipticities of galaxies from SDSS using casjobs?

texts: How to get ellipticities of a list of random galaxies. I saw few programs written in SQL and they were only giving stokes parameters and isophotes values for ONE galaxy(for which we input the ObjID). I am not familiar with syntax of SQL. Is there any method to put the objid in a loop so that the program generates stoke parameters values for a list of galaxies?


texts: Day,Night, and Year in the Ptolemaic Model

texts: As far as I understand, the Ptolemaic model explains day and night by postulating that the whole celestial system revolves around the Earth once every day. Since we observe the sun to move from East to West over a day, the whole system would have to move in an East to West direction once a day. But over the year, the planets move (usually) towards the East and the constellations towards the West. So in this model, the planets on their different move opposite to the daily motion of the whole system? Whereas the sphere of stars moves in the same direction as the daily motion of the whole system? I ask since I just found this odd for some reason.


texts: Can we calculate the area of sky visible to us from the point we are standing?I mean is there any idea or experiment to calculate it?


texts: Examples of radio correlations over times much longer than interferometric baselines?

texts:
Very Long Baseline Interferometry or VLBI such as the Event Horizon Telescope or JPL's delta-DOR (see below) uses baselines of the order of the Earth's diameter.

Spacecraft position determination
How do we track the exact location of the spacecraft which is millions or billions of miles away from us?
Help understanding BepiColombo's weak capture at Mercury's L1 and need for delta-differential one-way range measurements
Tracking Arc and Time for Precise Orbit Determination

The Earth's diameter is about 12,000 kilometers.

The Russian radio telescope satellite Spektr-R allows for an even larger baseline, with an apoapsis that can reach to 300,000 kilometers!

What artificial satellite has the farthest orbit around the Earth?
What are Spectr-R's major contributions to radio astronomy that could not have been done from Earth? (currently unanswered, should I move it here?)

These result in maximum arrival times of about 42 milliseconds and 1 second, respectively.

In order to combine these signals, mathematical correlation is used, and the "beam is steered" by varying the time delay in the correlation. This can be done in real time, or during "playback". In the case of the Event Horizon Telescope (from which we are waiting for a press conference next Wednesday) suitcases full of hard drives are flown on airplanes from the various sites and collected before the correlation can even begin.

How does the Event Horizon Telescope implement the interferometry?

Question: Have there been observations with much longer delays used in correlation of radio signals? Minutes? Months?


texts: Do all orbiting objects have barycenters?

texts:
From the simple perspective of someone like myself, it appears that our sun is "fixed in place" (from the perspective of the solar system itself) and that everything else of lower mass orbits around it.  I suspect that the sun wobbles due to the other objects (that the barycenter is either near it or within its radius).

It appears from reading about binary stars of more-or-less equal mass that they cannot have one "fixed in place" and the other orbiting it.  The barycenter is basically between them.

Is that perspective correct?  Is it ever possible for a low mass star to be the "fixed in place" center with a large mass star orbiting it?  Or does the barycenter always shift toward the more massive object?


texts: What is the cause of all of these sharp, concentric rings around bright stars in this HST image?

texts:
ESA's HST page heic1819 — Photo Release; Hubble reveals cosmic Bat Shadow in the Serpent’s Tail is of course beautiful and stunning, but my eyes are drawn to the diffraction artifacts of the bright stars.

I'm assuming the crosses are due to four vanes supporting the secondary mirror, but are the tight concentric rings due to Hubble's large aperture, or it's smaller secondary mirror blocking that aperture, or something else, perhaps image processing?

Without a scale for reference, it's hard to get an angular frequency and compare to an Airy-like diffraction pattern to get a diameter, and that's where it gets more puzzling, because you need a narrow wavelength range to get a coherent oscillation for so many cycles (I think I can see perhaps 15 or more sharp, distinct rings), and to zeroth order stars are mostly black-body.

Is this a bit of a puzzle, or am I missing something obvious (e.g. filters)? Or both?

Here's a cropped, monochromed, ROI:

Further stretched in contrast and size:


texts: Why are tides high only on a full moon day?

texts:
As far as I've known, sea tides are higher on a Full moon day. And that is due to the "gravitational pull" of the moon.
But we know that moon is visible because of reflection of sun's rays. So, even on a new moon's day, the moon is still there, but it's not visible.
So, why doesn't it affect the gravity on earth?

Thanks in advance.


texts: In what timeframe is W26 expected to go supernova?

texts: All the news-articles say that it is expected to "eventually" explode. Which doesn't really tell me much. When is "eventually"? Also, considering this is the largest star in the known universe and 16k light-years away, will this be visible? Will it be dangerous?


texts: Can we calculate the orbit of exoplanets?

texts:
I'm not an astronomer, but the question came up and I'm interested if we are able (or how accurate we are able) to calculate the orbital parameters of an exoplanet. Since the transit method must yield some properties of its orbit, we must know some of it. I also found some information here regarding the measurement of the orbital plane (Can we know the orbital planes of extraterrestrial planetary bodies?).

But do we have a clear picture of its orbit? Like we have of the planets in our system?

Could we, for example, tell if the planet was born in this system or has been caught by it at some point (giving it a different orbit I suppose)?


texts:
In multiple video clips of the moon but also one of Jupiter, some kind of waves can be observed. Is this just the effect of the object moving behind layers of different refractive indexes (similar to what we observe at sunset) or is there a wave propagating through the atmosphere?

https://m.youtube.com/watch?v=xUGxysKSGEM

https://m.youtube.com/watch?v=EMSkd4Xhp0g

There is an answer on Quora, but I would love to see a more detailed answer, why we can see exactly these two rather sharp lines moving.


texts: Is there a difference between the terms 'elliptical' and 'elongated' for galaxies?

texts: While studying the Solar System, I found that some galaxies are either elliptical or elongated. What's the difference?


texts: Why does the convective core in an intermediate to high mass star shrink?

texts:
The image below shows the evolution of the hydrogen mass fraction profile for a 5 solar mass star in the main sequence. I would expect that the size of the convective core stays roughly constant as the hydrogen is been fused, leading to a final step-like profile when the core hydrogen is exhausted. However, all the literature and simulations show a clear slope in the final step-like profile which results from the shrinking of the convective core.

Does anyone have any insight about the reason behind the shrinking?

(source of the image: http://astro.if.ufrgs.br/evol/evolve/hansen/StellarEvolnDemo/m5z02evoln.html)

Edit 1:

Below is the evolution of the temperature profile for the same star. Feel free to comment on it.

Personally, I am surprised how little the temperatures change. Since the main nuclear reaction is the CNO cycle which scale as $~T^{16}$, I was expecting a much more violent change. However, the central temperature only increases in about 30% during the whole main sequence. Interesting.

(Source: http://astro.if.ufrgs.br/evol/evolve/hansen/StellarEvolnDemo/m5z02evoln.html)

Edit 2:

I thought a nice explanation could be in the Ledoux criterion for convection. This criterion states that chemical gradients have a stabilizing effect against convection (i.e., it impedes convection) which would lead to the conclusion that, at the interface between the radiative and convective zone, the radiative transport would take over. However, I simulate the evolution with and without the Ledoux criterion and in both cases the convective core shrinks.


texts: Is there any practical use for astronomy?

texts:
Although astronomy is very cool and the things we are learning are awesome, is there really any practical use to knowing the things we know about the universe?

Do other fields of science draw from the current tome of astronomical knowledge?


texts: Referring to this question, is there any information that can leave black holes? Are they causing a permanent information loss in our Universe?


texts:
I remember seeing in a video an instrument that was used to measure a star's radius with nothing more than the light it emitted.
The device worked by opening or closing its mechanism in increments, while watching a display. The display started out showing some shape, maybe some lines or a circle, with blurriness. The closest you could get to making the shape blurry-less would determine the instrument's approximation of the star's radius.
My question is, what is the name of this instrument?


texts: Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?

texts:
I was reading this overview article about the Gaia spacecraft and I saw the following statement:

These spectra provide radial velocity information that are used to study the kinematic and dynamic evolution of the Milky Way. Radial velocities are derived from three isolated Calcium lines at 849.8, 854.2 and 855.2nm. Other lines in the 847 to 874nm range can provide data on star composition, surface gravity, and metal abundance.

note: as pointed out in the comments below, the third line is at 866.2nm, not 855.2nm - this is suggested to be a typo - the '855' number also shows up on this ESA page.
The high resolution spectroscopy only seems to operate between 847 and 874nm, and "three isolated Calcium lines" are used to measure radial velocity.
Do all stars have enough calcium in their atmosphere to produce strong enough features to measure the radial velocity so accurately? I had thought that there are some stellar populations have very little besides hydrogen and helium in their atmosphere.
Are these always emission lines or absorption lines, or will there be some stars with one and some of the other? What fraction of stars simply won't have significant amounts of calcium?

above: Gaia's Radial Velocity Spectrometer from here, credit: ESA.

above: Gaia's Imaging system, including mirrors 4, 5 and 6, prisms, diffraction gratings, and CCD array, from here, credit: EADS Astrium.

above: Gaia's Optical Module, including Radial Velocity spectrometer (gratings) and a focal field corrector, from here, credit: SAS Astrium.


texts: Is there a retrograde star that passes closer to the galactic center than our Sun?

texts: Our Sun is moving in a prograde orbit around the galactic center with a distance of about 8 kpc.  This paper says there are many retrograde stars in the galactic halo 10-20 kpc from the galactic center.  Do any of these retrograde stars have an extremely eccentric orbit that would cause them to pass closer to the galactic center than our Sun?


texts: Calculating Right Ascension and Declination from Latitude / Longitude / Time

texts:
I have a latitude / longitude / time of a location on Earth, I need to calculate the Right Ascension / Declination that's overhead at the time provided.

So far I've found out that Right Ascension is the same as the sidereal time for the location but I haven't found any information about declination.

I'm trying to set my overhead viewing location in the web portal of World Wide Telescope.  It defaults to "now" but I want to show 10PM and there's no way (that I've found) to set time through the javascript interface.  I can however set Right Ascension and Declination, hence my question.


texts: How is the gravitational effect of galaxies outside of the visible universe on galaxies within the visible universe currently modeled?

texts:
Given currently accepted models of the universe,

How much mass is outside of the visible universe?
What is the gravitational effect of that mass on the visible universe?

The question How strong is the gravitational stretch we experience from the edge of the universe? is possibly related, but it's focused on some stretching effect which I'm not familiar with.

One answer to the related question How much of the Universe is invisible to us, and how does it affect our theories? states:

  there does not seem to be any large mass concentrations outside the
  observable universe, since these would be seen as bias for movement of
  galaxies and galaxy groups, which also supports that the rest of
  universe is at least very close to what we can observe directly

But this talks about the absence of large mass concentrations, while I'm asking more generically how mass outside the visible universe is reflected in current models.


texts: How to create 3D animation of particles position vs time in python?

texts:
I am trying to make a 3-D animation of stars' position vs time. I have the information of the stars' x, y and z co-ordinates over time. I was looking at the example of Matplotlib and did understood what I need to do but I don't want the connecting lines between different times. All I need is to show the position of the stars at different time and make a movie of it. This is the example I was looking at

https://matplotlib.org/gallery/animation/simple_3danim.html

After that I want to make it a bit complicated. I have the stars size and mass as well. I was wondering if I can change the size of the circles in the plot according to the radius and use different Colors for different mass range?


texts: First observation that the movement of a planet or asteroid in its orbit was affected by another planet?

texts:
Answers to the question How did Kepler determine the orbital period of Mars? describing careful observations centuries ago got me thinking.

Question: What was the first analysis of observations that directly demonstrated that the movement of a planet or asteroid in its orbit was affected by gravitational attraction by another planet?

I don't mean inferences based on the spacing of orbits, I'm thinking of detailed positional measurements; something like planet/asteroid A was X kilometers or Y arcseconds away from it's predicted position and it couldn't be accounted for if not for the gravitational effects of planet B.

I am not sure if First observation that the Sun and Jupiter (and friends) move around a common barycenter? is a completely separate question, or if this confirmation happened all at once, but I've currently asked it separately.


texts: How close can planets form to one another?

texts: With the NASA announcement today regarding the discovery of a system containing seven earth-sized planets (3 within the habitable zone), I wondered about the seemingly crowded conditions.  What principles guide the formation of planets and the distances between them?  Are there laws governing this that are well established?  How close have we observed two planets form?


texts:
Is this term official jargon/wording for 'Dark Matter'?
If so, why include the word "Fluid" if there is supposedly no viscosity in actual space. It would be like saying 'Hey, want to go swim in my 'gravel' pool?"
http://www.dailymail.co.uk/sciencetech/article-3013284/Is-dark-matter-FLUID-Mystery-particles-mapped-galaxy-clusters-results-suggest-not-particles-all.html


texts: Are hot stars like O-type stars entirely composed of helium?

texts:
Hot stars like O-type stars show no hydrogen in their spectra. Does this mean they are made entirely of helium?

Any explanation would be really helpful.


texts: What was the train of lights (30+ in a row) that went south toward north then disappeared tonight in Turnersville Texas?

texts: The lights were bright and looked like stars that moved together in unison in the formation of a Double forward slash Ex: // but nearer together made up of (I’m guessing from what I saw)at least 30+ lights/star things varying in size , The larger on the top forward end and smaller and spaced apart slightly more toward the bottom rear per row. I am sure it wasn’t the starlink satellite constellation, it was not plane lights it wasn’t a fire lantern Floating or led balloon(s) it was not the army base ft hood! What was it ? Did anyone else see it tonight? I’ve never seen such a thing ever before? It wasn’t the Orionoid meteor shower it wasn’t a shooting star. This thing wasn’t attached visibly to anything, just lights. And as it moved the shape brightness size everything other than its location never changed it did not even change altitude! then just disappeared like now you see it now you don’t. There was no fading flickering all at once it was just gone!


texts: Where can I find a database on the masses of different galaxies?

texts: I want to get the data for masses of galaxies, more information about them can only be helpful. Can anyone point me to a site where I can get this sort of information?


texts: In this image, where would the antumbra be?

texts: In this image where would the antumbra be?:


texts: A point of light moving between the stars

texts: I noticed you replied to a question before about this topic and i came across your answer. Way back in the 80s when i was in my teens i saw a point of light moving between the stars at a steady rate across the sky. It was about the same size and brightnesz of those visible. It was moving in such a was as it approached a star ir veered off left or right then continued on its original path across the night sky. I showed my sister who was older than me at the time. It kind of reminded me of a whiplash efect using gravity to maintain spèed. Mabie too much Star Trek, but you get the idea. I am not an astronomer so forgive my ignorance, but a plane or a satalite would not move in this manor would it? I am convinced to this day that it was not one of ours. Can you clarify this as it was definately avoiding the stars?


texts: Why is the track of the subgiant stage almost horizontal on the HR diagram?

texts:
It is stated that:

After the main sequence, as fusion weakens or stops in the core, outward radiation weakens. The helium core contracts and heats up. Gravitational energy is converted to thermal energy again!
The star will appear to cool slowly and will undergo a modest increase in luminosity. During this phase, the path the star will follow in the HR diagram is almost horizontal to the right of its position on the Main Sequence. Stars in this phase are usually referred to as subgiants.

But I still can't understand.


texts: What coordinate system is at rest relative to the center of the galaxy?

texts: I am given equatorial coordinates and I need to transform them into a coordinate system that is at rest relative to the center of the galaxy. The closest thing I could find is galactic coordinates which seems like they are fixed with the sun at the center. Wouldn't this meant that they are moving with the same velocity as the sun and they are rotating?


texts: How bright is the full Earth during the lunar midnight?

texts:
At lunar midnight (i.e. the new moon as seen from Earth), the Earth is in its full phase with its entire disk in sunlight, and it is the brightest object in the lunar sky. How bright is it, and how variable is its brightness?

More precisely, I'm interested in specific figures for the magnitude and in comparisons to similar objects. Would I be able to read in that light? To drive? Would I notice the variations in brightness? What are the main variations due to?


texts: How is it possible for a collision to be responsible for Uranus's axial tilt?

texts: I realize that the collision explanation is in any case highly (purely?) speculative, but I'm curious how it would work.  If Uranus is a ball of gas, why wouldn't any colliding object just pass through (or perhaps be destroyed/significantly damaged by the heat of friction)?  What would there be for the colliding object to hit?  Did Uranus have a significant solid mass at one time?  Is this a function of the speeds that would have been involved (sort of akin to skipping rocks on a pond, if that's a relevant analogy)?


texts: Can Doppler shift make colors of two stars at same distance different?

texts:
As you know, Doppler effect can change the wavelength of the light.
Suppose there are two stars at same distance and same temperature. If one star is receding and the other star is approaching, their colors would look different.
What I want to know is, how probable is it that two stars show different colors because of Doppler shift? In other words, if two stars at same distance have different colors, say blue and red, then is it safe to assume that two stars have different surface temperature?
Actually, this was the question from the final exam for Wave & Heat.

Q. Stars emit radiation whose spectrum is very similar to that of an ideal blackbody. Two stars, identical in size, are at the same distance from us. One of the stars appears reddish in color while the other one looks very blue. Choose the correct statement.
(a) The red star is hotter.
(b) The blue star is hotter.
(C) They could both have the same surface temperature.

I read that some stars near the galactic center moves very fast. http://en.wikipedia.org/wiki/S2_(star)
If we only consider Planck distribution, only (b) is possible. I'm wondering whether (c) is also a realistic case, or it is only conceptually possible.


texts: In all these years of listening, why hasn't SETI detected anything ? Is there no intelligent life out there or is it too far away to detect ?


texts: Photosphere Thickness and Pressure vs stellar type

texts:
Where can I find information on how the thickness and pressure of a stellar photosphere is expected to vary with star type (temperature). I, as an amateur, have attempted a "simple" analysis of spectral line profiles to estimate these properties and whilst it gives good results for the Sun I can find no data on other stars with which to test my analysis. My work can be found on my website
and I am particularly interested in values for the blue component of Albireo and for Altair.